Assembly and spectroscopic interrogation of large atomic and molecular clusters in helium droplets

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In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Andrey Vilesov of The University of Southern California, interrogates chemical reactions at temperatures close to absolute zero (-460 °F). At such low temperatures, the internal motions of molecules are greatly slowed, making them easier to study. In order to study chemical reactions at such low temperatures, the atoms or molecules must be placed in an environment that keeps them cold, but also allows them to interact with each other. Since most common liquids freeze at such low temperatures, the atoms and molecules dissolved in them are prevented from colliding and interacting with each other. The PI overcomes these challenges by using helium droplets, which are still liquid at absolute zero, as test tubes in his studies. In his previous studies, Professor Vilesov made an additional interesting observation, namely that tiny vortices (similar to whirlpools) present in the helium droplets help form wire-shaped particles just a few atoms across. The properties and stability of such particles made of different materials are systematically studied with state of the art laser and cryogenic techniques. This project focuses on the mechanisms of aggregation of atoms and molecules at an ultra low temperature of 0.4 K. Free superfluid helium droplets up to 1 micrometer in diameter are doped with atoms or molecules, which then form clusters inside the droplet. The structure, stability and energy levels of the clusters are interrogated by laser spectroscopy, transmission electron microscopy and x-ray scattering techniques. Quantum vortices present in the superfluid droplets attract the imbedded particles serving as a template for low temperature aggregation. The properties of the resulting nanometer-thin, track-shaped clusters are in the focus of this project. In addition, this proposal includes the development of an experimental technique for the exploration of low temperature ion-molecule reactions using infrared spectroscopy and monitoring of the solvation effects in small carbocations and protonated water clusters. The broader impacts of this work include potential societal benefits from exploring unconventional paths for making useful nanomaterials. This project provides opportunities for training of undergraduate and graduate students in STEM field by developing of the cutting edge physical experiments and preparation of the ensuing publications.